This invention concerns the manufacture of iron oxides. More particularly, it concerns a method of producing a crystalline, red iron oxide of low acicularity. The utility of the invention resides in the production of pigmentary iron oxides.
Iron oxide, either natural or synthetic, is a well-know pigment. Among the synthetic oxides are blacks, yellows, browns, tans, and reds. The black and brown oxides contain varying ratios of ferrous and ferric oxides. Yellow iron oxides are the monohydrate of ferric oxide (Fe.sub.2 O.sub.3.H.sub.2 O) (also written as FeOOH). Tan oxides are iron oxide pigments in which iron oxide is reacted with another metallic oxide to form a series of chemical compounds known as ferrites. Synthetic red oxides can be made by four different methods. "Ferrite reds" are made by dehydrating the yellow oxide, thus converting the ferric oxide monohydrate to Fe.sub.2 O.sub.3. Another method is to precipitate black or brown oxide and then calcine at red heat to the ferric oxide state. A third method makes roasted "copperas reds", by dehydrating copperas, (FeSO.sub.4.7H.sub.2 O) to the monohydrate (FeSO.sub.4.H.sub.2 O) amd then calcining the monohydrate to give red alpha Fe.sub.2 O.sub.3 (copperas red). The fourth method gives what is often referred to as "precipitated reds", made by direct precipitation from a solution of ferrous salts. The present invention is concerned with an improvement in this last-mentioned method.
In U.S. Pat. No. 2,935,379 (Martin), brown gammma ferric oxide is digested in an acidic, aqueous suspension containing a soluble ferrous salt, this process forming red, non-magnetic, alpha ferric oxide (precipitated red). In U.S. Pat. No. 2,618,571, red iron oxide is obtained by oxidizing an alkaline suspension of ferrous carbonate containing zinc carbonate and/or aluminum hydroxide. In U.S. Pat. No. 2,620,261, red iron oxide is formed by precipitating brown ferric hydroxide in the presence of zinc or copper compound. In U.S. Pat. No. 2,618,532, red iron oxide is prepared by aerating an aqueous alkaline suspension of ferrous carbonate and magnesium carbonate. I found that these, and other similar methods of preparing precipitated reds, result in a mixture of iron oxide particles having acicular crystals and some not fully developed red iron oxide crystals. This non-uniformity of product results in poor color development.